Further Chemical Analysis of Kenyan Polygonum Species

Abstract

Polygonum species enjoys a wide distribution In the highland regions of Kenya. Eleven species are reported as present in this country with four species among these being most common. The four include; P.senegalense Meisn, P.salicifolium Willd, P. pulchrum Blume and P.setosulum A. Rich. and have found use in traditional medicine. Cold extraction of the leaves of the four species, through washing technique with acetone, followed by thin layer chromatographic analysis of the respective extracts found P. senegalense as the richest in flavonoid aglycones. Unlike in the three others, the leaves of P. senegalense species are either covered with resinous material (glabrous) in which flavonoid aglycones are dissolved or are covered by quasi-crystalline farina (tomentose) which also contain an almost identical set of flavonoid aglycones as the former. In either case, or variations in between (leaves both tomentose and glabrous), the flavonoid aglycones are restricted to the leaf surface with younger leaves having higher concentration. Petroleum ether wash of the leaf surface led to a flavonoid concentrate which was completely toxic to Aedes aegyti mosquito larvae at 10 ppm. Furthermore, 10 ppm of such a solution sprayed on a sugar coated Whatman No.1 filter paper completely protected it from being fed on by Schistocerca gregaria (desert locust) previously starved for 24 hours. Broad fractionation of this petroleum ether wash concentrate on silica gel using solvents; petroleum ether (40 - 60), benzene, chloroform and methanol concentrated the activities in the chloroform fraction. Complete resolution of this fraction followed by spectroscopic analysis revealed several flavonoid aglycones, among them: 2' ,6'-dihydroxy-3' ,4'-dimethoxychalcone -2-/ 2',6'- dihydroxy-4'-methoxydihydrochalcone 2; 2',4'- dihydroxy-3',6'-dimethoxychalcone 4 2' ,6'-dihydroxy-4'-methoxychalcone 2 2' ,4'-dihydroxy-6'- methoxychalcone 6; and 2' ,5'-dihydroxy-3' ,4' ,6'-trimethoxychalcone 8. Fractional crystallization of the benzene fraction yielded 2'-hydroxy-4' ,6'-dimethoxychalcone and 5-hydroxy-7-methoxyflavanone 2. Acid hydrolysate of the methanol fraction Ii yielded 7-hydroxy-5,8- dimethoxyflavanone 10 and 7 hydroxy-5-methoxyflavanone Antifeedant and larvicidal bioassay tests on these compounds found compound 2 to be toxic to the Aedes aegypti mosquito larva at 0.8 ppm. Compound 1 was found to be phago-stimulatory to Schistocerca gregaria while compound 2 showed high toxicity at 2 ppm towards Aedes aegypti mosquito larvae. Compound A, previously reported as showing molluscicidal activity, was found to be toxic towards Aedes aegypti larvae at concentrations of about 2 ppm .